Use and Potential
Increasing global demand for food will drive continued agriculture intensification.
Intensification can be applied to all cropping systems, with varying degrees
of constraints because of economics and the availability of labor and technology.
Rates of residue return to soil are also influenced by potential alternative
uses as fodder and fuel. Intensification of systems with previously low use
of purchased inputs (e.g., fertilizer, improved varieties, pesticides) may involve
increased use of these inputs and/or intensive management using biological inputs
(e.g., crop rotations, cover crops, manures). Where the use of purchased inputs
is already high, intensification implies increased efficiency (and potentially
reduced use) of fertilizer, pesticide, and other inputs. The principal means
by which intensification influences soil carbon changes are through the amount
and quality of carbon returned to soil (via roots, crop residues, and manures)
and through water and nutrient influences on decomposition (Paustian et al.,
2000a). Agricultural intensification can occur on all or nearly all of the world's
existing cropland (1.6 Bha).

Current Knowledge and Scientific Uncertainties
The rates of SOC sequestration by agriculture intensification differ among soils
and ecoregions. The influence of these practices on productivity and soil properties,
including organic matter dynamics, has been studied for many decades; there
is an extensive body of research results and many well-documented, long-term
field studies around the world (Powlson et al., 1998). Uncertainties
remain, however, regarding the interactions between different practices (e.g.,
crop rotations, water table management, fertilization) for different soil and
climate conditions.

Methods
Rates of soil carbon sequestration can be established for predominant cropping/management
systems on the basis of long-term benchmark experiments, on-site sampling, and
modeling (e.g., Powlson et al., 1996; Eve et al., 2000). Annual
statistics on cropland area and crop production are available globally at the
country level (e.g., FAO, IGBP-DIS), and more detailed data on crop production
and the extent and distribution of the practices described above exist to varying
degrees for all Annex I countries and many non-Annex I countries. Several generalized
models of carbon cycling in agricultural systems exist (see Chapter
2). Quantification of soil carbon changes can be estimated, using models,
from the distribution of major cropping systems, data on production and residue
returns, and associated soil and climate information, and/or with soil sampling
designs. Scaling from local to regional to national levels can be done by using
a combination of climate and soil maps, management and yield data, modeling,
and geographic information systems (GIS).

Time Scale
These practices can increase soil carbon stocks for 25-50 years or until saturation
is reached.

Monitoring, Verifiability, and Transparency
The amount of new carbon sequestration and its residence time (turnover rate)
can be verified through ground truthing (on-site sampling) and well-calibrated
models. Periodic monitoring can be done by using benchmark sites where SOC content
and bulk density can be measured once every 5-10 years to a depth of 1 m. Because
of the stratification of SOC, soil samples need to be taken in small depth increments
in the surface layers. The practices to be used are well characterized.

Permanence
Most carbon in agricultural systems is in the soil and has residence times of
years to centuries (see Section 4.2).

Associated Impacts
Agriculture intensification has numerous ancillary benefits-the most important
of which is the increase and maintenance of food production. Environmental benefits
can include erosion control, water conservation, improved water quality, and
reduced siltation of reservoirs and waterways. Soil and water quality is adversely
affected by indiscriminate use of agriculture inputs and irrigation water. Where
intensification involves increased use of nitrogen fertilizers, fossil energy
use will increase, as may N2O emissions.

Relationship to IPCC Guidelines
These practices relate directly to the "Input Factors" used in the IPCC Guidelines
for estimates of changes in soil carbon stocks. Default values for three levels
of plant residue production and addition to soil are provided in the Workbook,
with examples describing the types of management systems that each level would
correspond to.